Compact and low-cost visible laser sources with Watt-level power outputs have long been desired, especially for consumer electronics applications. One application that would be enabled by such laser sources is projection displays, either in rear-projection (TV) or front-projection format. The advantages of laser sources over traditional, bulb-based sources, are numerous. They include wider wavelength gamut, longer lifetime, higher throughput efficiency of the projection light engine due to low etendue and linear polarization of the laser light source.
Of the three basic colors (red, green, and blue, or RGB) required for projection display applications, green color has been the most challenging. The reason is the lack of commercially viable semiconductor materials that can provide direct lasing in the green wavelength. The most efficient semiconductor lasers rely on material systems such as InGa(Al)P that lase most efficiently in the near-infrared spectral region. In the visible spectrum, efficient operation down to ˜650 nm in the red wavelength regime can be achieved and semiconductor laser designs can be extended down to ˜635 nm with somewhat decreased efficiency and reliability. On the shorter wavelength side, GaN-bases material systems have been under intense development in recent years and semiconductor laser products in the violet (˜400 nm to ˜445 nm) spectral range have been commercialized. For these lasers, achieving longer wavelengths i.e., >470 nm in an efficient and reliable way represents a nearly insurmountable challenge. Thus, the majority of the visible spectrum (between ˜470 nm blue and ˜635 nm red) does not have an efficient semiconductor laser solution.
In non-consumer-electronics (scientific, instrumentation, etc) or lower-power applications (laser pointers), green laser sources have been developed using a diode-pumped solid-state laser platform with nonlinear frequency doubling. Since the 1990s, the indirect solution for the green laser source, available in non-consumer-electronics (scientific, instrumentation, etc) or lower-power applications (laser pointers) has been based on nonlinear frequency doubling (also known as second-harmonic generation, or SHG) of neodymium (Nd)-based solid-state lasers, such as Nd:YAG or Nd:YVO4. These solid-state gain materials can be pumped by infrared semiconductor lasers (e.g., at ˜808 nm) to produce laser radiation at ˜1064 nm wavelength. This radiation can then be frequency doubled into the green 532 nm wavelength using such nonlinear crystals as KTP or LBO. Similar technique can be used to obtain the blue wavelength such, e.g. 473 nm by frequency-doubling a 946 nm solid-state laser. A review of such approaches can be found in the book by W. P. Risk, T. R. Gosnell and A. V. Nurmikko, “Compact Blue-Green Lasers”, Cambridge University Press (2003).
The main obstacle in bringing the frequency-doubled, diode-pumped solid-state laser platform to mass manufacturing is its high cost and lack of scalability. While low-power green lasers (e.g., laser pointers) can be obtained with relative ease, scaling power to achieve Watt-level output typically requires costly cavity designs with multiple optical components, cooling modules with separate temperature controls, and large package dimensions.
Several ideas to overcome these limitations have been proposed. One concept relies on optically pumping a thin layer of semiconductor gain material, instead of a solid-state laser material, and intracavity frequency doubling the infrared laser radiation into the visible wavelength spectrum (A. Caprara, J. L. Chilla, L. A. Spinelli, “Intracavity frequency-converted optically-pumped semiconductor laser,” U.S. Pat. No. 6,167,068). This optically pumped semiconductor laser platform allows obtaining high power output in the blue-green spectral range, but still requires traditional, high-cost laser cavity architectures. This is a serious obstacle in using this platform in mass manufacturing. Another approach is using electrically pumped vertical extended cavity semiconductor lasers or laser arrays (Published US Patent Application 2006/0,029,120). The manufacturability of this platform is higher compared to the optically pumped lasers, because of the integrated nature of the pump and the gain in a single chip. However, this integrated nature is also a limiting factor and it makes it more difficult for electrically pumped lasers to reach the high power and high efficiency levels required in mass-producible devices for consumer markets.
Comparing the surface-emitting semiconductor laser and solid-state laser platforms for their use in producing green or blue wavelengths via nonlinear frequency doubling (or second-harmonic generation, frequently abbreviated as SHG), one can conclude that the solid-state laser platform is advantageous due to its higher gain, especially at the 1064 nm wavelength that can be readily converted into 532 nm green via SHG. However, increasing output power of the visible solid-state laser source while keeping the platform simple and low-cost has proven to be a much bigger challenge than it is for surface-emitting semiconductor laser.
One solid-state laser platform that could address both issues of mass manufacturability at low cost and scaling to high power levels is the so-called microchip laser platform. Originally described in the early 1990s (U.S. Pat. No. 5,365,539), this platform offers a compact, alignment-free laser cavity which comprises the gain crystal (such as Nd:YAG) and the nonlinear crystal, such as KTP (Potassium Titanium Oxide Phosphate, or KTiOPO4). The microchip laser platform is manufacturable at low-cost. It can also be scaled to high power levels by extending the microchip concept from a single emitter to multiple emitters, i.e. laser array. This was also described in U.S. Pat. No. 5,115,445, and later, in Published US Patent Application 2002/0,186,731.
However, despite the described advantages of these laser sources, their introduction to large-volume consumer-electronic markets did not happen, except at very low power levels as in laser pointers. One reason for this is the low efficiency of the overall green laser module (although hereinafter we will generally be referring to the green wavelength of 532 nm wavelength for description clarity, most of the description also applies to other frequency-doubled wavelengths, such as 473 nm blue).
Indeed, most low-cost single-emitter green laser sources have electrical-to-optical conversion efficiencies on the order of a few percent. Ignoring the power requirements of cooling modules, one can make the following illustrative estimate. It is known to use 808 nm edge-emitting lasers producing several hundreds of milliwatts (say, 300-500 mW) to produce only about 5-10 mW of green wavelength with a Nd:YVO4 gain crystal and a KTP nonlinear crystal. Thus, even the optical-to-optical conversion efficiency is generally in the range of 1-3%. Assuming 50% power conversion efficiency for the pump diode, we find that the overall electrical-to-optical efficiency is on the order of 1-2%. Trying to scale such low-efficiency single-emitter laser sources to achieve a several-Watt output power could result in ˜100 W of heat generated per 1 W of visible output. Several problems will immediately arise due to such inefficiency. One is keeping the platform compact and simple, since one will necessarily have to isolate the emitters sufficiently for best thermal performance. Another is dissipating such large amounts of heat without high-cost means such as using large chillers, etc.
Analyzing the root causes of low efficiency, one can understand that the conventional platform based on nonlinear materials such as KTP (more efficient) or LBO (less efficient) lacks the high conversion efficiency into the second-harmonic wavelength (e.g., green). Attempts to solve this problem were made by increasing the fundamental laser power and still using the KTP crystal. One example of such work can be found in the paper by Y-F. Chen, T-M. Huang, C-L. Wang, and L-J. Lee, “Compact and efficient 3.2-W diode-pumped Nd:YVO4/KTP green laser,” Applied Optics, vol. 37, p. 5727 (1998). The paper reported optical-to-optical (808 nm to 532 nm) conversion efficiency of 25%. However, one can also conclude that this was achieved in a platform that had to give up the advantages of low-cost and scalability. Some examples are fiber-coupled pump diode output (high-cost), curved mirror requiring active alignment (high cost and not scalable to arrays), cooling of crystal to 17° C. (high cost).
Another problem, which prevents this platform from providing a reliable product, is the low reliability of KTP crystals at the high power density levels in the green wavelength range. It is known that KTP crystals develop high-absorption areas in and around the green beam propagation path and therefore this phenomenon does not allow the use of KTP in high-power applications. The bulk nonlinear material that is normally used for high power green laser sources is LBO, but its nonlinear conversion efficiency is an order of magnitude lower than KTP's conversion efficiency. Therefore, LBO and other low-efficiency bulk nonlinear materials are not useful for low-cost, high-power applications.
In summary, despite the many advantages of solid-state lasers, conventional solutions for frequency-doubled visible solid-state laser sources cannot be engineered for high-power, low-cost, manufacturable, and reliable consumer-electronics products. The engineering problems are fundamental and multiple design goals (primarily, high power, low cost, and reliability) cannot be met at the same time. It must also be noted that the low efficiency limitation mentioned earlier is also a big factor for a surface-emitting semiconductor laser platform, which is typically less efficient compared to the 1064 nm solid-state laser platform.